Method for washing prefoamed particles

ABSTRACT

A method of washing pre-expanded particles having a substantially water insoluble inorganic compound attached thereto by contacting the pre-expanded particles with an aqueous washing solution of a water soluble compound. The water soluble compound has a solubility with water of at least 1 g/100 g of water and has 1-500 surface bonding functional groups and 1-500 hydrophilic functional groups. At least one of the surface bonding functional groups of the water soluble compound attaches to the water insoluble inorganic compound.

TECHNICAL FIELD

The present invention relates to a method for washing pre-expandedparticles. More particularly, the present invention relates to a methodfor washing pre-expanded particles which are used for production of athermoplastic resin internal die expansion molded article which is usedas a cushioning and wrapping material, a carrying box, an adiabaticmaterial, a bumper core of an automobile and the like.

BACKGROUND ART

It has been known that thermoplastic resin pre-expanded particles can beprepared by a method (hereinafter referred to as “pressure releaseexpansion method”) comprising dispersing thermoplastic resin particlesinto water containing a dispersant in a pressure container, adding avolatile blowing agent thereto, impregnating the thermoplastic resinparticles with the volatile blowing agent with keeping high temperatureand high pressure, and releasing the particles into a lower pressureatmosphere.

In the above pressure release expansion method, in order to preventfusion between resin particles at high temperature and high pressure, ingeneral, there are used a slightly water soluble inorganic compound(hereinafter sometimes referred to as “inorganic dispersant”) as adispersant and a surface active agent (hereinafter sometimes referred toas “auxiliary dispersant”). However, even after the resin particles athigh temperature and high pressure are released into a lower pressureatmosphere to give pre-expanded particles, part of the inorganicdispersant remains on and attaches to the pre-expanded particles surface(hereinafter, the inorganic dispersant which remains on and attaches tothe pre-expanded particles is sometimes referred to as “attachingdispersant”). If the amount of the attaching dispersant is large, fusionbetween the pre-expanded particles becomes bad when a mold is chargedwith the pre-expanded particles and the particles are heated with vaporto give an internal die expansion molded article.

In order to solve the above problem, in general, there are employed amethod (A) comprising washing the pre-expanded particles with a largeamount of water or warm water, and a method (B) comprising washing thepre-expanded particles with an acidic aqueous solution.

However, in the method (A), washing effect is poor, so that theattaching dispersant cannot be sufficiently removed. Also because alarge amount of water is used, the method (A) is uneconomical.

On the other hand, in the method (B), because an equipment must be acidresistant, cost of the equipment becomes high. Also, in the case thatthe inorganic dispersant is insoluble in an acid, the method (B) is noteffective.

Furthermore, waste water after washing is suitably treated anddischarged. However, in the method (A), because a large amount of wastewater must be treated, cost of the treatment becomes high. In the method(B), because the inorganic dispersant is water-solubilized, steps suchas a reprecipitation step and an adsorption and removal step are needed.Accordingly, cost becomes high in addition to complication of the steps.

In order to provide a method for efficiently washing and removing theattaching dispersant without an acid resistant equipment, the inventorsof the present invention have earnestly studied. As a result, it hasbeen found that by washing pre-expanded particles prepared by thepressure release expansion method with an aqueous solution of a watersoluble compound having a specific functional group in its molecule, theabove problems can be solved. Then, the present invention has beenaccomplished.

DISCLOSURE OF THE INVENTION

The present invention relates to

{circle around (1)} a method for washing pre-expanded particles preparedby charging a pressure container with an aqueous dispersion containingthermoplastic resin particles, a slightly water soluble inorganiccompound as a dispersant and a surface active agent, and a volatileblowing agent, increasing the temperature to reach the constant pressureand the constant temperature, impregnating the thermoplastic resinparticles with the volatile blowing agent, and releasing the particlesinto a lower pressure atmosphere,

which is characterized in that

the pre-expanded particles are washed with an aqueous solution of awater soluble compound having at least one functional group capable ofbonding to the surface of the slightly water soluble inorganic compoundand at least one hydrophilic functional group in its molecule;

{circle around (2)} the above method wherein the slightly water solubleinorganic compound is tricalcium phosphate, and the water solublecompound is a water soluble phosphate, a water soluble condensatedphosphate or a mixture thereof;

{circle around (3)} the above method wherein the water solublecondensated phosphate is sodium hexametaphosphate;

{circle around (4)} the above method wherein the slightly water solubleinorganic compound is tricalcium phosphate, and the water solublecompound is a water soluble silicate;

{circle around (5)} the above method wherein the slightly water solubleinorganic compound is tricalcium phosphate, and the water solublecompound is a water soluble ethylenediamineteraacetate;

{circle around (6)} the above method wherein the slightly water solubleinorganic compound is tricalcium phosphate, and the water solublecompound is a water soluble citrate, a water soluble tartrate or amixture thereof;

{circle around (7)} the above method wherein the slightly water solubleinorganic compound is tricalcium phosphate, and the water solublecompound is a sodium polyacrylate; and

{circle around (8)} the above method wherein the slightly water solubleinorganic compound is kaolin, and the water soluble compound isethylenediaminehydrochloride.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, the pre-expanded particles are prepared by aconventionally known method, that is, the method (pressure releaseexpansion method) comprising charging a pressure container with anaqueous dispersion containing thermoplastic resin particles, a slightlywater soluble inorganic compound (inorganic dispersant) as a dispersantand a surface active agent (auxiliary dispersant), and a volatileblowing agent, increasing the temperature to reach the constant pressureand the constant temperature, impregnating the thermoplastic resinparticles with the volatile blowing agent, and releasing the particlesinto a lower pressure atmosphere.

In the above pressure release expansion method, in order to preventfusion between the thermoplastic resin particles in the pressurecontainer, the inorganic dispersant and the auxiliary dispersant areused.

As the inorganic dispersant, conventionally used compounds can be usedwithout particular limitation. Concrete examples of the inorganicdispersant are, for instance, an inorganic salt such as tricalciumphosphate, magnesium carbonate or calcium carbonate; clay such asbentonite or kaolin; and the like. Among them, from the viewpoint thatdispersing power is great, tricalcium phosphate is preferable. From theviewpoint that dispersing power is great and no environment pollutionoccurs, kaolin is preferable. In general, the surface of the inorganicdispersant is charged with positive or negative electricity.

The above auxiliary dispersant is a component used for improvingaffinity between the thermoplastic resin particles and the inorganicdispersant and heightening dispersing power. As the auxiliarydispersant, conventionally used compounds can be used without particularlimitation. Concrete examples of the auxiliary dispersant are, forinstance, an anionic surface active agent such as sodiumdodecylbenzenesulfonate, sodium n-paraffinsulfonate, sodium higheralcohol sulfate or sodium alkylnaphthalene sulfonate; a cationic surfaceactive agent such as benzalkonium chloride, alkyltrimethylammoniumchloride or dialkyldimethylammonium chloride; and the like. Among them,from the viewpoint that great dispersing power is imparted andbiodegradation easily occurs, sodium n-paraffinsulfonate is preferable.From the viewpoint that great dispersing power is imparted, benzalkoniumchloride is preferable.

As the auxiliary dispersant, in general, a surface active agent which ischarged with reverse electricity to the surface charge of the inorganicdispersant is used. For instance, in the case that tricalcium phosphateof which surface is charged with positive electricity is used as theinorganic dispersant, in general, the anionic surface active agent suchas sodium n-paraffinsulfonate or sodium dodecylbenzenesulfonate is used.In the case that kaolin of which surface is charged with negativeelectricity is used as the inorganic dispersant, in general, thecationic surface active agent such as benzalkonium chloride ordialkyldimethylammonium chloride is used. Among them, from the viewpointthat dispersing power is great, the combination of tricalcium phosphateand sodium n-paraffinsulfonate is preferable.

As mentioned above, in general, the combination of the inorganicdispersant and the auxiliary dispersant which is charged with reverseelectricity to the charge of the inorganic dispersant is employed. As aresult, a hydrophilic group (ionic group) of the auxiliary dispersantmutually functions with the surface of the inorganic dispersant, thesurface of the inorganic dispersant becomes hydrophobic by a hydrophobicgroup of the auxiliary dispersant, and mutual function power of thesurface of the inorganic dispersant with the surface of the hydrophobicthermoplastic resin particles is improved. That is, the auxiliarydispersant functions as a binder between the thermoplastic resinparticles and the inorganic dispersant, so that dispersing power isheightened.

In the present invention, the pre-expanded particles prepared by thepressure release expansion method are washed. The particles are washedwith the aqueous solution of a water soluble compound having at leastone functional group capable of bonding to the surface of the inorganicdispersant and at least one hydrophilic functional group in itsmolecule. The above water soluble compound is a compound of whichsolubility with water at ordinal temperature is at least 1 g/100 g(water), preferably at least about 5 g/100 g (water).

The capability of bonding to the surface of the inorganic dispersantshows existence of reverse ionicity to ionicity on the surface of theinorganic dispersant, and is capability of ionic bonding. The functionalgroup capable of bonding to the surface of the inorganic dispersant is afunctional group showing reverse ionicity to ionicity on the surface ofthe inorganic dispersant.

The above hydrophilic functional group is a group such as a polar groupor a dissociated group, which weakly bonds to water molecule byelectrostatic mutual function or hydrogen bond, and which shows affinitywith water.

Concretely, for instance, in the case that tricalcium phosphate of whichsurface is charged with positive electricity is used as the inorganicdispersant, as a washing agent, there is used an aqueous solution of awater soluble compound having a functional group capable of bonding tothe surface of the inorganic dispersant, such as an anionic functionalgroup such as phosphoric acid group (ion), carboxyl group (ion) orsilicic acid ion in its molecule, and having a hydrophilic functionalgroup such as phosphoric acid group (ion), carboxyl group (ion), silicicacid ion or alcoholic hydroxy group in its molecule. The functionalgroup capable of bonding to the surface of the inorganic dispersant maybe the same as or different from the hydrophilic functional group.Concrete examples of the water soluble compound are, for instance, awater soluble condensated phosphate such as sodium pyrophosphate, sodiumtripolyphosphate or sodium hexametaphosphate; a water soluble phosphatesuch as trisodium phosphate or tripotassium phosphate; a mixture of thewater soluble condensated phosphate and the water soluble phosphate; awater soluble silicate such as sodium metasilicate or sodiumorthosilicate; a water soluble ethylenediaminetetraacetate such asdisodium ethylenediaminetetraacetate or tetrasodiumethylenediaminetetraacetate; a water soluble citrate such as sodiumcitrate; a water soluble tartrate such as sodium tartrate; a mixture ofthe water soluble citrate and the water soluble tartrate (water solublehydroxycarboxylate); a sodium polyacrylate; and the like. These can beused alone or in admixture thereof. Among them, from the viewpoint thatwashing effect is more great, the water soluble condensated phosphate ispreferable. From the viewpoint that pH of the aqueous solution becomes7.5 or so and handling of the aqueous solution is easy, sodiumhexametaphosphate is particularly preferable.

For instance, in the case that kaolin of which surface is charged withnegative electricity is used as the inorganic dispersant, as a washingagent, there is used an aqueous solution of a water soluble compoundhaving a functional group capable of bonding to the surface of theinorganic dispersant, such as a cationic functional group such as aminogroup (ion), quaternary ammonium ion or pyridinium group in itsmolecule, and having a hydrophilic functional group such as amino group(ion), quaternary ammonium ion, pyridinium group or alcoholic hydroxygroup in its molecule. The functional group capable of bonding to thesurface of the inorganic dispersant may be the same as or different fromthe hydrophilic functional group. Concrete examples of the water solublecompound are, for instance, a water soluble polyvinylpyridinium compoundsuch as poly-4-vinyl-N-ethylpyridinium bromide;polyoxyethylenealkylamine; an ethylenediaminetetraacetate such asdisodium ethylenediaminetetraacetate or tetrasodiumethylenediaminetetraacetate; ethylenediaminehydrochloride; and the like.These can be used alone or in admixture thereof. Among them, from theviewpoint that washing effect is more great,ethylenediaminehydrochloride is preferable.

When the pre-expanded particles are washed with the aqueous solution ofthe water soluble compound having at least one, preferably 2 to 500functional groups capable of bonding to the surface of the inorganicdispersant in its molecule, and having at least one, preferably 2 to 500hydrophilic functional groups in its molecule, the amount of theattaching dispersant decreases. In addition, because pH of the aqueoussolution of the water soluble compound, used for washing is, in general,6 to 11 (the aqueous solution showing higher pH can be used because pHis adjusted to 7 to 8 with an acid), no acid resistant equipment whichhas been conventionally used for washing with an acidic aqueous solutionis needed. Also, there are employed no conventionally needed steps forwaste liquid in washing with an acidic aqueous solution, such as aneutralization and reprecipitation step, an aggregation andprecipitation step, and as occasion demands, an adsorption and removalstep (this step is needed, because particle size of precipitatedparticles which have been neutralized and reprecipitated becomes small,so that it is difficult for the particles to aggregate and precipitate).That is, waste liquid in washing can be treated by only the aggregationand precipitation step, and steps are simplified and cost becomes low.

For the reason about decrease of the amount of the attaching dispersantin accordance with the above washing, it is considered that because thefunctional group of the water soluble compound in the aqueous solutionas the washing agent bonds to the surface of the inorganic dispersantand hydrophilic property is imparted to the surface of the inorganicdispersant by the hydrophilic group in the washing agent, binder effectof the auxiliary dispersant is weakened and the amount of the attachingdispersant decreases.

The stage for washing the pre-expanded particles is not particularlylimited. From the viewpoint that washing effect becomes great and thewashing step is hardly lengthened, it is desired that the particles arewashed just after pre-expansion.

The concentration of the water soluble compound in the washing agentvaries according to the kinds and the amount of the inorganic dispersantand the auxiliary dispersant. In general, the concentration ispreferably 5 to 500 ppm, more preferably 20 to 300 ppm. The amount ofthe washing agent is preferably 5 to 40 L per 1 kg of the pre-expandedparticles, more preferably 10 to 30 L per 1 kg of the pre-expandedparticles. From the viewpoint that dispersion about expansion ratio ofthe pre-expanded particles can be decreased, it is desired that thetemperature of the washing agent is about 50° to 80° C.

Because the amount of the washing agent is greatly less than the amountof water or warm water, that is, 40 to 80 L per 1 kg of the pre-expandedparticles in washing with water or warm water, the method of the presentinvention is economical and also, waste water in washing can be easilytreated.

Examples of the method for washing the pre-expanded particles with thewashing agent are, for instance, a method comprising blowing the washingagent to the pre-expanded particles through a shower nozzle, a methodcomprising immersing the pre-expanded particles in the washing agent,and the like. The method is not limited to the exemplified ones.

The pre-expanded particles prepared by the above pressure releaseexpansion method are washed with the aqueous solution of the watersoluble compound having at least one functional group capable of bondingto the surface of the inorganic dispersant and at least one hydrophilicfunctional group in its molecule, so that the amount of the attachingdispersant can be decreased to 300 to 600 ppm, from 1000 to 2000 ppm.

The pre-expanded particles applied for the method of the presentinvention are, as mentioned above, particles which have been prepared byimpregnating the aqueous dispersion in which the thermoplastic resinparticles have been dispersed in water with the inorganic dispersant andthe auxiliary dispersant, with the volatile blowing agent at hightemperature and high pressure, and then, pre-expanding the thermoplasticresin particles by the pressure release expansion method.

Examples of a thermoplastic resin from which the above thermoplasticresin particles are made are, for instance, low density polyethylene,medium density polyethylene, high density polyethylene, linear lowdensity polyethylene, polypropylene homopolymer, ethylene-propylenerandom copolymer, propylene-butene random copolymer,ethylene-propylene-butene random copolymer, polybutene, polystyrene andthe like. These can be used alone or in admixture thereof. It ispossible that to the thermoplastic resin are added a cell nucleatingagent, an antioxidant, a weather resistant, an antistatic agent, a flameretardant, a lubricant, a crystal nucleating agent and the like to giveresin particles.

The amount of the inorganic dispersant and the auxiliary dispersantvaries according to the kinds thereof, and the kinds and the amount ofthe thermoplastic resin particles. In general, it is desired that theamount of the inorganic dispersant is about 0.2 to 3 parts by weight(hereinafter referred to as “parts(s)”) based on 100 parts of water, andthe amount of the auxiliary dispersant is about 0.001 to 0.1 part basedon 100 parts of water.

Typical examples of the above volatile blowing agent are, for instance,isobutane, n-butane, n-pentane, propane, carbon dioxide, nitrogen andthe like. The volatile blowing agent is not limited to the exemplifiedones. It is desired that the amount of the volatile blowing agent isabout 5 to 50 parts based on 100 parts of the thermoplastic resinparticles.

The temperature and pressure during the pressure release expansion byimpregnating the thermoplastic resin particles with the volatile blowingagent vary according to the kinds of the thermoplastic resin particlesand the expansion ratio of the aimed pre-expanded particles. Thetemperature may be at lowest softening point of the thermoplastic resin.In general, it is desired that the temperature is about 100° to 150° C.,and the pressure is about 5 to 30 kg/cm²·G. Also, it is desired that theexpansion ratio of the pre-expanded particles is about 5 to 40 times.

The equipment (pressure container) is not particularly limited. Theequipments which can resist the pressure and the temperature in thepre-expansion step may be used. Concrete examples of the equipment are,for instance, an autoclave type pressure container and the like.

As preferable embodiment of the method of the present invention, therecan be cited

a method comprising

charging the autoclave type pressure container with 100 parts of apolyolefin resin particles (melting point: Tm° C. ), 150 to 500 parts ofwater, 0.2 to 3 parts of tricalcium phosphate as the inorganicdispersant, 0.001 to 0.1 part of sodium n-paraffinsulfonate as theauxiliary dispersant and 5 to 30 parts of isobutane as the volatileblowing agent,

increasing the temperature of the contents,

impregnating the polyolefin resin particles with isobutane at theconstant temperature within the range of (Tm−10)° to (Tm+10)° C. and theconstant pressure within the range of 10 to 30 kg/cm²·G,

releasing the polyolefin resin particles into atmospheric pressure togive pre-expanded particles, and

immediately washing the pre-expanded particles by blowing an aqueoussolution (50° to 80° C. ) thereon, containing 20 to 300 ppm of sodiumhexametaphosphate, in an amount of 10 to 30 L per 1 kg of thepre-expanded particles, through the shower nozzle.

The method for washing pre-expanded particles of the present inventionis more specifically explained by means of the following Examples, andit is to be understood that the present invention is not limited to theExamples.

EXAMPLES 1 TO 12 AND COMPARATIVE EXAMPLES 1 TO 7

The autoclave type pressure container was charged with 100 parts ofpellet of ethylene-propylene random copolymer (melting point: 149° C.,weight of one particle: about 1.8 mg), 300 parts of water, 1.0 part ofthe inorganic dispersant shown in Table 1, 0.01 part of the auxiliarydispersant shown in Table 1 and 12.0 parts of isobutane. The temperatureof the contents was increased to 148.5° C. with stirring and kept for 20minutes. Then, by additionally press fitting isobutane thereto, theinternal pressure of the container was adjusted to 19.3 kg/cm²·G andkept for 10 minutes. Then, with keeping the temperature and the internalpressure of the container constant by press fitting isobutane thereto, avalve in the lower part of the pressure container was opened and anaqueous dispersion was released into atmospheric pressure through anorifice (open diameter: 4 mmφ) to give pre-expanded particles. Justafter the aqueous dispersion was released through the orifice to givethe pre-expanded particles, the pre-expanded particles were showerwashed with a washing liquid (60° C.) shown in Table 1 at aconcentration and in a liquid amount per 1 kg of the pre-expandedparticles shown in Table 1. The expansion ratio of the washedpre-expanded particles was 24 to 26 times.

After the washed pre-expanded particles were dried at 70° C. in ahot-air drier, the amount of the attaching dispersant was measured inaccordance with the following method. The results are shown in Table 1.

Waste water is treated by a method comprising adding a precipitatingagent such as aluminum ion, trivalent iron ion or a high molecularaggregating agent to waste water, aggregating a dispersant and carryingout precipitation and filtration. In washing according to the method ofthe present invention, the amount of water to be used can be decreasedin comparison with washing with warm water. Accordingly, the amount ofthe precipitating agent and necessary time for treating waste water(treating power for waste water of equipment) can be saved.

On the other hand, in washing with an acidic aqueous solution, areprecipitation step by neutralization is firstly needed. Then, the sameaggregation and precipitation step as in the above washing with warmwater and in the above washing according to the method of the presentinvention is carried out. In this case, in addition to necessity of theextra reprecipitation step, fine particles are easily prepared in thereprecipitation step by neutralization, so that a larger amount of theprecipitating agent is needed in the later aggregation and precipitationstep.

(Amount of attaching dispersant)

{circle around (1)} Inorganic dispersant: Tricalcium phosphate

To a conical beaker were collected and added 50.0 mL of an aqueoussolution (colorimetric solution) containing 0.022% by weight(hereinafter referred to as “%”) of ammonium metavanadate, 0.54% ofammonium molybdate and 3% of nitric acid, and “W” (g) of thepre-expanded particles. After these were stirred for 1 minute, thesewere allowed to stand for 10 minutes. The obtained aqueous solution(coloring solution) was poured into a fused cell having an optical pathlength of 1.0 cm, and then, absorbance “A” at a wavelength of 410 nm wasmeasured by a spectrophotometer.

As to the same colorimetric solution, by using previously measuredabsorptivity “ε” (g/L·cm) of tricalcium phosphate at a wavelength of 410nm, the amount of the attaching dispersant “X” (ppm) was calculated inaccordance with the following equation:${X({ppm})} = \frac{5.0 \times {10^{4} \cdot ɛ \cdot A}}{W}$

{circle around (2)} Inorganic dispersant: Kaolin

To a crucible were collected and added “Wr” (g) of the pre-expandedparticles, and the pre-expanded particles were heated by using a gasburner till the particles completely became to be ashes. Then, theweight of the residual ashes “Wa” (g) was measured. By the same manneras this, using “Wp” (g) of the resin particles which had not beenpre-expanded yet, the weight of the residual ashes of these resinparticles “Wb” (g) was measured. Using the measured values, the amountof the attaching dispersant “X” (ppm) was calculated in accordance withthe following equation:${X({ppm})} = {\left\lbrack {\frac{Wa}{Wr} - \frac{Wb}{Wp}} \right\rbrack \times 10^{6}}$

TABLE 1 Washing liquid Liquid amount Amount of Aqueous dispersionConcen- (L/kg (Pre-) attaching Ex. Inorganic Auxiliary tration expandeddispersant No. dispersant dispersant Kinds (ppm) particles)) (ppm) 1Tricalcium Sodium n- Trisodium phosphate aqueous 50 30 480 phosphateparaffinsulfonate solution 2 Tricalcium Sodium n- Sodiumhexametaphosphate 15 30 320 phosphate paraffinsulfonate aqueous solution3 Tricalcium Sodium n- Sodium hexametaphosphate 50 30 300 phosphateparaffinsulfonate aqueous solution 4 Tricalcium Sodium n- Sodiumhexametaphosphate 100  30 280 phosphate paraffinsulfonate aqueoussolution 5 Tricalcium Sodium n- Sodium hexametaphosphate 50 15 360phosphate paraffinsulfonate aqueous solution 6 Tricalcium Sodium n-Sodium pyrophosphate aqueous 50 30 320 phosphate paraffinsulfonatesolution 7 Tricalcium Sodium n- Sodium tripolyphosphate aqueous 50 30350 phosphate paraffinsulfonate solution 8 Tricalcium Sodium n- Sodiummetasilicate aqueous 50 30 520 phosphate paraffinsulfonate solution 9Tricalcium Sodium n- Disodium 50 30 430 phosphate paraffinsulfonateethylenediaminetetraacetate aqueous solution 10 Tricalcium Sodium n-Sodium citrate aqueous solution 50 30 520 phosphate paraffinsulfonate 11Tricalcium Sodium n- Sodium polyacrylate aqueous 50 30 540 phosphateparaffinsulfonate solution 12 Kaolin Benzalkonium Ethylenediaminehydrochloride 50 30 580 chloride aqueous solution Com. Tricalcium Sodiumn- Warm water — 15 1960  Ex. phosphate paraffinsulfonate 1 2 TricalciumSodium n- Warm water — 15 1960  phosphate paraffinsulfonate 3 TricalciumSodium n- Warm water — 60 1000  phosphate paraffinsulfonate 4 TricalciumSodium n- Sodium acetate aqueous solution 50 30 1180  phosphateparaffinsulfonate 5 Tricalcium Sodium n- Glycerin aqueous solution 50 301200  phosphate paraffinsulfonate 6 Kaolin Benzalkonium Warm water — 301820  chloride 7 Kaolin Benzalkonium Ammonium chloride aqueous 50 301570  chloride solution

From the results shown in Table 1, it can be understood that in Examples1 to 12 according to the method of the present invention, in comparisonwith Comparative Examples 1 to 7 according to the conventional method,the amount of the attaching dispersant is extremely small such as 280 to580 ppm.

TEST EXAMPLES 1 TO 3 AND COMPARATIVE TEST EXAMPLES 1 TO 3

To a pressure container were added each pre-expanded particles preparedin Example 3 or Comparative Example 2, and the particles were compressedat an air pressure of 2.5 kg/cm²·G. A mold (internal length: 50 mm×300mm×300 mm) was compression filled with the compressed particles andthen, molding was carried out by using vapor having a molding heatpressure shown in Table 2. After the obtained molded article was driedat 80° C. for 12 hours in an oven, the dried molded article wasgradually cooled to room temperature. Density of the molded article wascalculated and tensile elongation of the molded article was calculatedin accordance with the following method. The results are shown in Table2.

(Tensile elongation)

The molded article was cut into a dumb-bell test piece (thickness:10±0.5 mm, width of parallel part: 10±0.5 mm, length between markedlines: 50±0.5 mm) so that no surface layer of the molded articleremained. After the dumb-bell test piece was allowed to stand in a roomat 20° C. for at least 24 hours, tension test was carried out under thecondition at tensile speed of 100 mm/min. and measuring temperature of20° C. At breakpoint, migration length “L” (cm) between marked lines wasmeasured, and tensile elongation was calculated in accordance with thefollowing equation:${{Tensile}\quad {elongation}\quad (\%)} = {\frac{L - {Lo}}{Lo} \times 100}$

wherein “Lo” (cm) was length between marked lines before measuring.

Thus calculated tensile elongation is a barometer of fusion betweenpre-expanded particles.

TABLE 2 Molding Density of heat molded Tensile Pre-expanded pressurearticle elongation particles (kgf/cm² · G) (g/L) (%) Test Example No. 1Example 3 3.8 46 11.3 2 Example 3 4.0 45 14.9 3 Example 3 4.2 45 16.4Comparative Test Example 1 Comparative 3.8 46  8.1 Example 2 2Comparative 4.0 45 10.1 Example 2 3 Comparative 4.2 45 11.1 Example 2

From the results shown in Table 2, it can be understood that when themolded article is produced as in Test Examples 1 to 3, by using thepre-expanded particles of Example 3, which have been washed inaccordance with the method of the present invention, tensile elongationis large and short fusion between the pre-expanded particles hardlyoccurs.

INDUSTRIAL APPLICABILITY

When the pre-expanded particles prepared by the pressure releaseexpansion method are washed according to the method of the presentinvention, the amount of the attaching inorganic dispersant can bedecreased by using the same equipment as in washing with water or warmwater and without consuming a large amount of water or warm water. As aresult, an internal die expansion molded article having excellentproperties can be produced.

What is claimed is:
 1. A method for washing pre-expanded particles, theparticles prepared by a) charging a container with an aqueous dispersioncontaining thermoplastic resin particles, a substantially waterinsoluble inorganic compound as a dispersant and a surface active agent,and a blowing agent; b) increasing the temperature of the container to aconstant temperature; c) maintaining the constant temperature and aconstant pressure in the container so as to impregnate the thermoplasticresin particles with the blowing agent; and d) releasing the particlesinto an atmosphere having a lower pressure than the constant pressure ofthe container to form pre-expanded particles, wherein said substantiallywater insoluble inorganic compound attaches to said pre-espandedparticles, the method comprising: a) providing an aqueous washingsolution of a water soluble compound, said water soluble compound havinga solubility with water of at least 1 g/100 g of water and having 1-500surface bonding functional groups which bond to a surface of thesubstantially water insoluble inorganic compound and 1-500 hydrophilicfunctional groups, said aqueous washing solution having a pH of 6 to 11;and b) washing said pre-expanded particles by contacting saidpre-expanded particles having said substantially water insolubleinorganic compound with said aqueous washing solution, wherein one ofsaid surface bonding functional groups of said water soluble compoundbonds to the surface of said substantially water insoluble inorganiccompound attached to said pre-expanded particles.
 2. The method of claim1, wherein the substantially water insoluble inorganic compound istricalcium phosphate, and the water soluble compound is a water solublephosphate, a water soluble condensated phosphate or a mixture thereof.3. The method of claim 1, wherein the substantially water insolubleinorganic compound is tricalcium phosphate, and the water solublecompound is a water soluble silicate.
 4. The method of claim 1, whereinthe substantially water insoluble inorganic compound is tricalciumphosphate, and the water soluble compound is a water solubleethylenediaminetetraacetate.
 5. The method of claim 1, wherein thesubstantially water insoluble inorganic compound is tricalciumphosphate, and the water soluble compound is a water soluble citrate, awater soluble tartrate or a mixture thereof.
 6. The method of claim 1,wherein the substantially water insoluble inorganic compound istricalcium phosphate, and the water soluble compound is sodiumpolyacrylate.
 7. The method of claim 1, wherein the substantially waterinsoluble inorganic compound is kaolin, and the water soluble compoundis ethylenediaminehydrochloride.
 8. The method of claim 1, wherein thesubstantially water insoluble inorganic compound is selected from thegroup consisting of tricalcium phosphate and kaolin.
 9. The method ofclaim 1, wherein the water soluble compound is selected from the groupconsisting of a water soluble phosphate, a water soluble condensatedphosphate, a mixture of the water soluble phosphate and the watersoluble condensated phosphate, a water soluble silicate, a water solubleethylenediaminetetraacetate, a water soluble citrate, a water solubletartrate, a mixture of the water soluble citrate and the water solubletartrate, a sodium polyacrylate, and ethylenediaminehydrochloride. 10.The method of claim 1, wherein the substantially water insolubleinorganic compound is selected from the group consisting of tricalciumphosphate and kaolin; and the water soluble compound is selected fromthe group consisting of a water soluble phosphate, a water solublecondensated phosphate, a mixture of the water soluble phosphate and thewater soluble condensated phosphate, a water soluble silicate, a watersoluble ethylenediaminetetraacetate, a water soluble citrate, a watersoluble tartrate, a mixture of the water soluble citrate and watersoluble tartrate, a sodium polyacrylate, andethylenediaminehydrochloride.
 11. The method of claim 2, wherein thewater soluble condensated phosphate is sodium hexametaphosphate.